Railways generally consist of a track supported by a bed of crushed rock or gravel, this bed being referred to as “ballast” in the art. Most tracks comprise a pair of rails fixed to lateral members known as “sleepers” (or in some countries, “cross-ties”). Sleepers are formed from steel, timber or concrete.
There is a need as part of normal railway maintenance for the removal of track ballast. This is usually brought about by the ballast becoming fouled with soil or the like, or through there being excessive ballast present as a result of past maintenance. Ballast fouling prevents escape of water that can inundate the railway during heavy rain or flooding, while excessive ballast can lead to instability of the railway.
Ballast removal requires lifting of the track so that the ballast between and underneath the sleepers can be accessed. Normally, ballast removal is necessary over many kilometres of railway. The process must therefore be done mechanically as manual removal would be too inefficient due to the amount of labour that would be required to rapidly carry out the process to minimise track closure time.
Apparatus for removal of ballast is known in the art. One such apparatus is a sled consisting of a plough that is V-shaped in plan and is mounted into a frame over which the track is passed as the sled is drawn therebeneath. Skids are included in the frame to allow passage of the track over the sled which sled is pulled by steel cables connected between the sled frame and a locomotive. Ballast removal using this apparatus is referred to a “track sledding”.
There are a number of disadvantages in using the sled described in the preceding paragraph for ballast removal. First, the cables by which the sled is pulled can break creating a potentially serious safety hazard. Second, track components, particularly sleepers, can be damaged as the track slides over the sled. Third, there is insufficient control over the sled during the sledding operation. Consequently, the remaining ballast bed may have a poor surface and the resulting track alignment poor necessitating substantial additional resurfacing.
Other apparatus for removal of ballast utilises endless chains to cut the ballast out from the track. Apparatus of this type leaves the track with good alignment and surface. However, the apparatus has a high capital cost of manufacture and is slow yet expensive to operate. Such apparatus is known in the art as ballast undercutting machinery.
Because of the disadvantages of ballast undercutting machinery, track sledding apparatus is the apparatus of choice for ballast removal. There is nevertheless a need for track sledding apparatus that overcomes the disadvantages of the apparatus described above. Specifically, there is a need for a track sledding machine that meets the following requirements:                1. the plough or any equivalent component is sufficiently controlled so that the surface of the remaining ballast or formation after the sledding operation is close to the original surface or to a desired surface with respect to top and cant without the need for substantial resurfacing thereby providing a railway that is suitable for the immediate passage of trains;        2. the track alignment can be sufficiently controlled during ballast removal to minimise resurfacing alignment corrections so that the railway is again suitable for the immediate passage of trains,        3. damage to track is eliminated or at least minimised; and        4. it can be moved at a speed that permits efficient removal of ballast without compromising operator safety.        